Electronic microscope



Feb. 10, 1942. I

E. RUSKA ELECTRONIC MICROSCOPE Filed Aug. 20, 1940 INVENTOR. die/varEms/(,4.

ATTORNEYS.

Patented Feb. 10, 1942 u ELECTRONIC MICROSCOPE Ernst Buska,Berlin-Spandau, Germany, asslgnor to "Fides Gesellschatt fiir dieVerwaltung und Verwertung von gewerblichen Schutzrechten mitbeschriinkter Hattung, Berlin, Germany, a corporation of GermanyApplication August 20, 1940, Serial No. 353,338 In Germany July 22, 19397 Claims. (Cl. 250-495) This invention relates to improvements inelectronic microscopes.

In many cases it is important that electron optical apparatus beprovided with as small an electron emitting source as possible. This is,for instance, the case if the rays are to pass through the objects to bemagnified as convergent rays. To this end, the cathode must be of anextremely small size and the rays must be emitted at an angle which isnot too small. When employing such a cathode, each point of the objectis struck by an approximately needleshaped beam of rays which convergestowards the center of the objective diaphragm and passes through alarger zone of the object than corresponds to the irradiated zone of theobjective diaphragm. The main object of the invention is to provideelectron-optical apparatus, in particular electron microscopes, with acathode which fulfills all the conditions mentioned. above. According tothe invention, the electron source of a microscopic or otherelectron-optical apparatus consists of a pointed or needle-shaped coldcathode formed, for instance, of a tungsten filament. Aside from thefact that such a cathode dispenses with the heat sources which areotherwise necessary, the arrangement may be so designed that anadditional control electrode as well as the means for producing thecontrol voltage are also eliminated.

The invention may be carried into practice in various ways. Thus, forinstance, the pointed cathode may be arranged in a support impressedwith the same voltage as the cathode proper and so designed that itshields the lines of force which emerge laterally from the pointedcathode which otherwise could pass to the anode. A particuilarly goodshielding efifect is attained if an auxiliary electrode is employedwhich is impressed with a negative voltage as compared with that of thecathode.

A pointed cathode as an electron emitting source of electron-opticalapparatus may be employed to advantage in various types of electronicmicroscopes. For instance, the cathode may be employed in electronicmicroscopes in which the electron rays emitting from the cathode areconverged by means of a condenser lens onto the object and after havingpassed through the latter are subjected to one or more electron lensesserving to project a magnified image of the object onto a luminescentscreen or photographic plate. The invention may also be applied to anarrangement in which the object is arranged more closely to the cathode,for instance directly in front of the cathode. According to anotherembodiment, the beam of electron rays emitted from the pointed cathodeis first concentrated by means of one or more electron optical reductionlenses to a' very fine luminous point, behind or in front of which theobject to be magnified is arranged in the path of the rays. In thelast-mentioned case a diaphragm with a very fine aperture is placed inthe neighborhood of the luminous point for the production of an imagewith augmented contrasts.

In the accompanying drawing are shown various embodiments of theinvention in diagrammatic form.

Figs. 1 to 3 show three different examples of a pointed cathodeaccording to the invention,

Figs. 4 to 6 show three different forms of electronic microscopes.

Referring to the drawing, l denotes a pointed cathode which is held inposition by a support 2. 3 denotes the anode and 4- the diaphragmthrough whose aperture the cathode rays pass to strike the object (notshown). As will be seen from Fig. 1, a comparatively large number oflines of force emerge laterally from the pointed cathode and pass to theanode. An arrangement in which the lines of force are more concentratedon the aperture of the diaphragm 4 is shown in Fig. 2. This effect isbrought about by giving the support 6 for the pointed cathode aparticular shape so that it contains the needleshaped cathode l within acup-shaped cavity. A still greater concentration of the lines of forceon the diaphragm aperture 4 is obtained if a control electrode 1according to Fig. 3 is employed and impressed with a negative potentialas compared with that of the cathode l.

Fig. 4 is a longitudinal sectional view of an electronic microscope inwhich the rays pass through the object and thence through an objectivelens. ll denotes the pointed cold cathode of the electron emittingsource. From this cathode a beam of electron rays is emitted at anaperture angle 20:5. The electron rays are converged with the aid of acondenser coil 12 and pass through the diaphragm I3 carrying the object.In the path of ray is also arranged an objective coil It provided withan objective diaphragm |5.- A fluorescent screen or photographic platefor producing an intermediate image is indicated at I5.

a indicates the distance between the electron source and the condenserlens, b the distance between the condenser lens and the objectivediaphragm I5, I) the distance between the objective diaphragm and theoptical plane of the image produced by the objective lens, and a thedistance of the-object from the objective diaphragm. F denotes the focallength of the objective lens. If dk denotes the diameter of the cathode,do the diameter of the object field. and do}! the diameter of theobjective diaphragm, the following equations are applicable:

I I d =2g,a,az2%a,f

As will be seen from Fig. 4, the point at which the electron rays areconverged by the condenser coil l2 lies in the opening of the objectivediaphragm I5.

When using a pointed cathode as the electron emitting source in thearrangement described above, the radius of curvature of the cathodepoint in the case of a tungsten unicrystal filament amounts, forinstance, to 10- mm., 1. e., dk==2 10- mm. The aperture a (free offaults) of a magnet objective amounts to about 10- With a focal lengthof the objective lens of fo=5 mm. the objective diaphragm should have adiameter doB=2aojo=2'10 5=0.01 mm. For the projection of the cathode inthe aperture of the objective diaphragm a magnification must thereforebe chosen. If the aperture of the pointed cathode amounts to as=0.2 theaperture of the beam when passing through the objective diaphragm is asfollows:

phragm 28 with a fine opening for the production of an image with strongcontrasts. The fiuorescent screen or photographic plate is arranged asindicated at 21. Fig. 5 also shows part of the vacuum vessel of themicroscope formed of the vessel portions 29, 30 and 3| and of the metalbodies of lenses 23 and 24.

A very simple electron optical magnifying device also containing apointed cathode according to the invention is shown in Fig. 6. The electronic rays emitting from the pointed cathode 3| pass directly throughthe object 32 which is projected on a fluorescent screen 33, or on aphotographic plate which may replace the screen.

What is claimed is 1. In an electron-optical apparatus having a sourceof an electron beam, means for holding an object in the path of saidbeam, and electronoptical lens means for causing said beam to produce amagnified image of the object, said source havinga cold cathode ofpointed shape directed towards the object.

2. In an electron-optical apparatus 'having a source of an electronbeam, means for holding an object in the path of said beam, andelectronoptical lens means for causing said beam to produce a magnifiedimage of the object, said source having a needle-shaped cathode, ananode diaphragm having an opening opposite the point of said cathode,and a conductive body surrounding the shaft portion of saidneedle-shaped cathode to shield the lines oi force emerging laterallyfrom said cathode.

3. In an electron-optical apparatus having a source of an electron beam,means for holding an object in the path of said beam, andelectronoptical lens means for causing said beam to produce a magnifiedimage of the object, said source having a cold cathode of pointed shapedirected towards the object, an anode having an opening opposite thepoint of said cathode, and a conductive support carrying said cathodeand forming a cavity surrounding said cathode to shield said anode fromlines of force emerging laterally from said cathode.

4. In an electron-optical apparatus having a source of an electron beam,means for holding an object in the path of said beam. andelectronoptical lens means for causing said beam to produce a magnifiedimage of the object, said source having a needle-shaped cold cathode, ananode having an opening'opposite the point of said cathode, and anelectrode having a negative potential as regards said cathode andsurrounding said cathode to shieldsaid anode from lines of forceemerging laterally from said cathode.

5. An electron microscope, comprising a source of an electron beamhaving a cold cathode oi pointed shape and an anode diaphragm with anopening opposite the point of said cathode, a condensing lens, an objectdiaphragm, an objective lens having an objective diaphragm, and acondensing lens arranged between said source and said object diaphragmand designed for concentrating said beam so as to taper through saidobject diaphragm to a focal point lying substantially in the plane ofsaid objective diaphragm.

6. An electron microscope, comprising a source of an electron beamhaving a cold cathode of pointed shape and an anode diaphragm with anopening opposite the point oi said cathode. diaphragm means for holdingan object, said means being arranged immediately in front of saidelectron source, and electron-optical means for causing the beam toproduce a. magnified image of the object.

7. An electron microscope, comprising a source of an electron beamhaving a cold cathode of pointed shape and an anode diaphragm with anopening opposite the point 01' said cathode, electron-optical reductionlens means for concentrating the beam to form a fine beam pointobject-holding means arranged so as to place the object in proximity tosaid beam point, and means arranged behind the obect its magnifiedimage. J for renewing ERNST RUSKA.

